X-ray images in grid-pattern format are typically produced by using X-ray detectors that have a plurality of pixel elements. Each pixel element serves to obtain a relevant item of information which is assigned to the associated pixel in a grid pattern.
A single item of information, namely a gray-scale value, has hitherto be obtained that is a measure of the extent to which X-rays emanating from an X-ray source are attenuated by an object situated between the X-ray source and the X-ray detector's respective pixel element.
A widely employed X-ray detector includes photodiodes on which a scintillator material is arranged. A scintillator layer converts X-ray quanta into light quanta, specifically converting each X-ray quantum into a plurality of light quanta, with the total energy of the light quanta being in scale with the energy of the X-ray quanta. The photodiodes register the total energy of all light quanta impinging upon them. A corresponding measured value keeps increasing light quantum for light quantum. The sum (integral) of the light-quantum energies and hence of the X-ray-quantum energies is thus obtained as the measured value for each pixel element.
A body permeated by X-rays will attenuate X-ray quanta of different energies to a differing extent. Because an X-ray source usually has a broadband spectrum, an item of information about how individual frequency bands from the spectrum are attenuated would be desirable. Materials can be assigned to regions in the body being X-rayed once said information is available, for example it can be determined whether a region consists of bone material or metal, and different metals can be distinguished etc.
For recognizing different materials a plurality of images recorded with different X-ray spectra have been used hitherto in X-ray systems as employed, for example, for checks at airports.
A current development trend aims at constructing X-ray detectors in such a way as already to possess an energy-resolving capability. Relevant concepts to date are based on converters that convert the X-rays directly into electrons. Converters of such kind based on, for instance, cadmium telluride (CdTe) are difficult to produce, can be provided covering a small area only, and costly tiling appears necessary.